A solid oxide fuel cell (SOFC) is a device that generates electricity by a chemical reaction. FIG. 1 shows a conventional SOFC assembly including a plurality of stacked “cells” in which each cell includes an anode layer 102, a cathode layer 104, and an electrolyte layer 106. Fuel cells are typically characterized by their electrolyte material, with SOFCs having a solid oxide or ceramic electrolyte.
During operation of the SOFC, an oxidant, usually air, is fed through a plurality of air channels 108 defined by the cathode 104, while fuel, such as hydrogen gas (H2) is fed through a plurality of fuel channels 110 defined by the anode 102. Typically, the oxidant and fuel channels are oriented at right angles to one another. The anode and cathode layers are separate by an electrolyte layer 106. During operation, the oxidant is reduced to oxygen ions at the cathode. These oxygen ions can then diffuse through the solid oxide electrolyte to the anode, where they can electrochemically oxidize the fuel. In this reaction, a water byproduct is given off as well as two electrons. These electrons are transported through the anode to an external circuit (not shown) and then back to the cathode, providing a source of electrical energy in the external circuit.
The flow of electrons in the external circuit typically provides an electrical potential on the order of approximately one volt. To generate larger voltages, fuel cells are typically arranged in “stacks” composed of a larger number of individual cells with an “interconnect” joining and conducting current through immediately adjacent cells. The stack design of FIG. 1 is a flat-plate or “planar” SOFC, in which three separate “cells” are shown arranged in a repeating sequence. The adjacent cells are separated by an interconnect 112, which serves to connect each cell in series so that the electricity each cell generates can be combined.
Although planar SOFC configurations have a number of advantages over other types of fuel cells, it is challenging to provide adequate sealing to prevent fuel-oxidant mixing and to electrically-insulate the stack. Seal leakage can lead to inefficient device performance (including fuel cell failure), costly device maintenance, and safety related issues. In planar SOFCs, the sealant is in contact with all other components of the cell and thus is subject to stringent requirements. Suitable sealing materials must be non-conducting and be able to function at the very high operating temperatures of SOFCs (typically 800-850° C.) and to withstand both oxidizing and reducing environments as well as reaction gases generated inside the SOFC during operation.
The sealing material must be able to survive extended service at elevated temperatures and repeated thermal cycles. If the sealing material expands at a rate that is different than the thermal expansion rate of the cell components, the sealing material may either crack or cause cracking of the cell components. As a result, the thermal expansion coefficient (CTE) of a seal material and stack components are kept as close as possible to avoid thermal stresses between sealant and cell during the SOFC operation.
Glass-ceramics are among the most promising sealants because, by controlling the crystallization of glasses (i.e., the nature, shape, and volume fraction of crystals), the CTE of the material can be tuned to match the CTEs of the cell components, such as, for example, yttria-stabilized zirconia (YSZ), lanthanum strontium titanate (LST), lanthanum strontium manganite (LSM), and nickel oxide-YSZ composite. Moreover, glass-ceramics exhibit mechanical robustness, long term stability at cell operating temperatures, electrically insulating behavior, good wetting of cell components, and ready application to the surfaces to be sealed as glass-frit powder dispersed in a paste, or as a tape-cast sheet that subsequently is subjected to thermal treatments of sintering and crystallization.
US Pat. App. No. 2011/0200909 by Parihar et al. for “Thin, Fine Grained and Fully Dense Glass-Ceramic Seal for SOFC Stack,” which is assigned to the assignee of the present invention, teaches a seal including a Sanbornite (BaO.2SiO2) crystal phase, a Hexacelsian (BaO.Al2O3.2SiO2) crystal phase, and a residual glass phase. While increasing the sanbornite content of glass ceramics leads to a consequent increase in CTE, which can be used to provide a good match to a SOFC stack, parent glasses having high sanbornite contents after crystallization do not show optimal sintering behavior, which can result in porous seal layers after thermal treatment (sintering+crystallizing).
Therefore, there is a need for an improved glass-ceramic seal for a solid oxide fuel cell stack.